Modern cameras frequently include optical systems, sometimes called zoom lenses, having multiple or infinitely variable focal lengths ranging from wide-angle to telephoto. It is common in such systems to include groups of optical elements that are moveable relative to each other along the optical axis to change the focal length. It is desirable at the same time to maintain focus throughout the entire range of focal lengths. This usually is accomplished by moving the optical elements relative to the film or other imaging plane.
There are many techniques for controlling movement of the optical elements in a zoom lens system to maintain focus while changing focal length. One such technique is illustrated in U.S. Pat. No. 4,971,427, filed in the name of Takamara et al. and issued Nov. 20, 1990. The Takamara et al. patent discloses a camera having front and back lens groups and a bell crank coupled therebetween to vary the inter-group spacing and thereby change the focal length. Both lens groups are retained in a lens barrel that is movable along an optical axis relative to the film plane. Movement of the lens barrel causes the bell crank to engage and follow a cam plate that rotates the bell crank and shifts the back lens group along the optical axis relative to the front lens group.
Another technique is disclosed in U.S. Pat. No. 4,391,496, filed in the name of Schilling et al. and issued Jul. 5, 1983. According to the Shilling disclosure, an elbow linkage couples first and second lens groups in a pancratic objective system. The linkage includes a cam follower at the elbow for engaging and following a cam surface so that axial movement of the first lens group extends or contracts the linkage, changing the distance between the lens groups and thereby the system focus.